PositionStage
function yields the position structure associated with
a particular
stage of a CEG.
PositionStage(stage.vector, num.category, pos.next.level)
(vector) - a set of situations that constitute a particular stage
(numeric) - number of edges that unfolds from the situations
(vector) - It identifies the positions corresponding to all situations that are children of situations associated with the variable spanning our target stage.
list of vector - Each vector identifies a position.